Neodymium stable isotopes in rivers and the oceans

Overview

Radiogenic neodymium (Nd) isotopes are widely used to trace water mass movement in the oceans, and have yielded remarkable insights into the relationship between ocean circulation, weathering, and climate change [e.g. 1]. The behaviour of Nd in seawater is well understood, and it is generally thought that Nd is largelly derived from the weathering of the continental rocks surrounding the ocean basins [e.g. 2]. However, it has long been recognised that the Nd content of the oceans cannot be accounted for by the different dissolved sources to the oceans (rivers and hydrothermal input) or by dust input [3]. It has been proposed that this missing Nd must be sourced by the exchange or weathering of Nd from river borne particulates in estuaries or from sediments on continental shelves, both involving exchange at the continental margins (so-called “boundary exchange”) [4].
The primary source of particulate material to the oceans is riverine transport. The combined riverine particulate flux, from suspended sediment and bedload, far exceeds elemental input from of dust and the dissolved riverine load, consequently the particulate phase dominates the transport of most elements to the oceans). The dissolution of even a small proportion of this material can therefore have a large impact on the chemical composition of seawater, with the effects being most significant for insoluble elements such as the Rare Earth Elements (REE) including Nd. Isotope measurements of particulates from the Amazon estuary suggest that they rapid release Nd to the estuary waters [5], consistent with dissolution experiments on particulate material, from rivers and estuaries, in seawater, indicating that up to substantial particulate Nd is released in a matter of days, sufficient to dominate the dissolved Nd [6]. However, some experiments suggest that Nd is actually adsorbed onto particulates rather than released, and the behaviour of Nd cannot be elucidated by radiogenic isotopes alone.

Neodymium stable isotopes are potentially controlled by a number of factors during weathering, like radiogenic isotopes they vary with source rock composition, but unlike radiogenic isotopes they are also influenced by the dissolution and precipitation of primary and secondary minerals, adsorption onto colloids or particles, and complexation with organic and inorganic ligands.

This project aims to measure Nd stable isotopes in rivers, estuarine environments and seawater in Scotland and West Greenland (Figure 1), with the aim of using the stable isotopes of Nd to determine (i) the behaviour of this element during weathering, (ii) trace the erosional transport of Nd, and (iii) quantify the nature of the interaction of particulate Nd with seawater.

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Image Captions

Kangerlussuaq fjord, West Greenland

Methodology

This project will use state-of-the-art chemical and analytical analytical techniques for the measurement of Nd stable isotopes in river, estuary and seawater samples to achieve the research aims outlined above.

River waters, suspended sediments, and estuarine waters will be collected in Scotland and North West Greenland or Norway, and the project will also have access to unique sample archives of rivers and seawater collected previously.

Project Timeline

Year 1

Training in the chemical and analytical procedures for the measurement of Nd stable isotopes, including isotope analysis by TIMS; fieldwork in Scotland; write/ defend Research Proposal;

Year 2

Sample and data processing; fieldwork in Greenland or Norway, develop writing and presentation skills, involving manuscript preparation and conference presentation.

Year 3

Synthesise field and isotope datasets; attend international conferences; publication and thesis writing.

Year 3.5

Complete and submit thesis; finalise manuscripts for publication.

Training
& Skills

Training in the measurement of Nd stable isotopes using high precision TIMS techniques at Durham, as well as water characterisation.

Fieldwork in Greenland (ancient shield terrain) or Norway (mixed rock terrain) and Scotland (mixed rock types).

Interpretation and modelling of isotope and elemental data to place new constraints on the behaviour of Nd in the weathering environment

Presentation of research at both national and international geochemistry conferences.

References & further reading

[1] Piotrowski, A.M., Goldstein, S.L., Hemming, S.R. and Fairbanks, R.G., 2004. Intensification and variability of ocean thermohaline circulation through the last deglaciation. Earth and Planetary Science Letters, 225(1-2), pp.205-220.

[2] Goldstein, S.L. and Hemming, S.R., 2003. Long-lived isotopic tracers in oceanography, paleoceanography, and ice-sheet dynamics. Treatise on geochemistry, 6, p.625.

[3] Tachikawa, K., Athias, V. and Jeandel, C., 2003. Neodymium budget in the modern ocean and paleooceanographic implications. Journal of Geophysical Research: Oceans, 108(C8).

[4] Lacan, F. and Jeandel, C., 2005. Neodymium isotopes as a new tool for quantifying exchange fluxes at the continent-ocean interface. Earth and Planetary Science Letters, 232(3-4), pp.245-257.

[5] Rousseau, T.C., Sonke, J.E., Chmeleff, J., Van Beek, P., Souhaut, M., Boaventura, G., Seyler, P. and Jeandel, C., 2015. Rapid neodymium release to marine waters from lithogenic sediments in the Amazon estuary. Nature communications, 6, p.7592.

[6] Pearce, C.R., Jones, M.T., Oelkers, E.H., Pradoux, C. and Jeandel, C., 2013. The effect of particulate dissolution on the neodymium (Nd) isotope and Rare Earth Element (REE) composition of seawater. Earth and Planetary Science Letters, 369, pp.138-147.

Further Information

For further information please contact Kevin Burton (kevin.burton@durham.ac.uk)

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